An electric power steering apparatus (EPS) which provides a steering system of a vehicle with an assist torque by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b , a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the calculated current command value. A steering angle sensor 14 is not essential, it does not need to be provided, and it is possible to obtain the steering angle from a rotation sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 exchanging various information of a vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed Vel from the CAN 40. Further, it is also possible to connect a non-CAN 41 exchanging a communication, analog/digital signals, a radio wave or the like except with the CAN 40 to the control unit 30.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 on the basis of the steering torque Th and the vehicle speed Vel and by using an assist map or the like. The calculated current command value Iref1 is added in an adding section 32A to a compensation signal CM from a compensating section 34 for improving a characteristic. A current limiting section 33 limits a maximum value of the current command value Iref2 to which the compensation signal CM has been added. The current command value Irefm of which the maximum value has been limited is inputted into a subtracting section 32B, where a detected motor current value Im is subtracted from the current command value Irefm.
Proportional integral (PI) control to a deviation I (=Irefm−Im) which is the subtraction result in the subtracting section 32B is performed in a PI control section 35. The voltage control value Vref obtained by the PI control is inputted into a PWM control section 36, which calculates duty command values, and PWM-drives the motor 20 through an inverter circuit 37 by means of a PWM signal. The motor current value Im of the motor 20 is detected by a motor current detector 38, and is inputted and fed back to the subtracting section 32B. Further, a rotation sensor 21 such as a resolver is connected to the motor 20, and a steering angle θ is outputted.
The compensating section 34 adds a detected or estimated self-aligning torque (SAT) to an inertia compensation value 342 in an adding section 344, further, adds a convergence control value 341 to the addition result in an adding section 345, and outputs the addition result obtained in the adding section 345 as the compensation signal CM to the adding section 32A so as to improve a characteristic of the current command value.
Thus, the conventional electric power steering apparatus detects the steering torque that a driver manually adds as a twist torque of a torsion bar by means of the torque sensor, and mainly controls the motor current as an assist current corresponding to the torque. Therefore, the steering torque may vary in accordance with the steering angle by a difference of a state of a road surface (for example, a slope, a low μ road, etc.).